The labor ministry said Tuesday that the internal radiation exposure of 142 workers involved in emergency operations at the crippled Fukushima No. 1 nuclear power plant was higher than reported by plant operator Tokyo Electric Power Co.
After recalculating data provided by Tepco, the Health, Labor and Welfare Ministry found that a male employee was exposed to 180 millisieverts, exceeding the 100-millisievert level that is said to raise the risk of cancer. The man's exposure was initially reported to be around 90 millisieverts.
The recalculation also revealed that two other workers were exposed to radiation of 50 to less than 100 millisieverts.
The three will be entitled to receive cancer or other health checks in addition to regular medical examinations.
The 142 were among the 7,500 workers involved in crisis containment operations in March and April 2011. After the Fukushima No. 1 plant was hit by a huge earthquake and tsunami on March 11, three reactors suffered meltdowns and hydrogen explosions damaged three reactor buildings at the site.
Accurate radiation measurement was difficult shortly after the crisis began because Tepco did not have enough whole-body counters to determine internal exposure.
The labor ministry has therefore urged Tepco to assess workers' exposure by taking into account that radiation levels may have been more serious.
Last July, the ministry reviewed the data on around 1,300 workers and said the reported exposure levels of 452 were too low. The ministry has checked the data of the remaining 6,200 and made the latest announcement on Tuesday.
The ministry ordered Tepco the same day to improve its data management.